In manufacturing an electronic device by using the technology of growing a GaN film on a substrate such as sapphire, lattice mismatch due to the different lattice constants appears seriously at the interface between the substrate and the GaN film (see FIG. 5a). Dislocations due to such lattice mismatch are transferred into the GaN thin film, so that the performance of an electronic device such as an LED and a solar cell is significantly degraded. To improve the performance and reliability of an LED, it has been proposed to grow a GaN thin film having a lower defect density on a sapphire substrate.
To that end, the epitaxial lateral overgrowth (ELOG) technique has been developed in which a mask made of SiNx or SiO2 material is patterned on a sapphire substrate or a GaN thin film grown on the sapphire substrate, and then the GaN thin film is grown (see FIG. 5c).
According to the ELOG technique, the SiO2 mask blocks the transfer of defects in the GaN thin film caused between different materials, so that the threading dislocation defects has been drastically reduced to approximately 107 cm−2. However, there is a problem in that the process of growing crystal is divided into two steps, which increases chances of processing failure caused by contamination, etc.
To remedy such shortcoming of the ELOG technique, the patterned sapphire substrate (PSS) that has micrometer-size structures thereon have been widely used (see FIG. 5b). The PSS typically has periodic structures having a shape such as polygons or cones having a size of several μm, which are arranged in the four directions spaced apart from one another by a distance of several μm and may protrude or be depressed from the surface. The depth or height of the pattern is typically more or less 0.5 μm, and the pattern is formed by dry or wet etching.
The method for growing a GaN single crystal using the PSS can improve the light extraction efficiency by approximately 15% when applied to an LED, compared to a method using a typical flat sapphire substrate, since light is reflected in the interface of the substrate having depressions. However, it fails to improve the basic cause of the defect density. This is not because the defects in the GaN are reduced but because light is concentrated due to the reflection in the interface of the substrate. Accordingly, there is still a problem in the reliability of the optical device.
Further, according to the ELOG technique and the PSS, since SiO2 is deposited via a photolithography process and micro-patterned by dry or wet etching, the process becomes complicated and thus there are more chances of failure. In addition, the ELOG technique and the PSS cannot significantly reduce the threading dislocations generated in the vertical direction, which affect the performance of the device most.